This invention relates to arrangements for holding magnets on components such as rotors of electrical machines that rotate at high speed.
In conventional permanent magnet electrical machines, magnets are radially retained on a rotor by nonmagnetic caps which are keyed into adjacent pole pieces or by providing angled sides on the magnet which are radially retained by wedged-shaped adjacent laminated pole pieces. Such arrangements, however, do not provide sufficient strength to hold a magnet securely in position when subjected to the radial accelerations encountered in high speed machines. In such arrangements, moreover, the pole pieces are subjected to increased stress as a result of the added radial load of the magnet during high speed operation. Furthermore, when angled sides of a magnet are engaged by wedged-shaped adjacent pole pieces, the radial location of the magnet may change, causing imbalance in the rotor. Also, such magnet support arrangements introduce flux path discontinuities in the dovetails and oblique surfaces of the magnetic components which decreases the efficiency of the machine.
The Iwata U.S. Pat. No. 5,811,908 discloses U-shaped permanent magnet retention channels in which the outer ends of the channel walls have projections to be received in corresponding grooves in adjacent pole pieces and the entire channel is made of a single piece of magnetic material. In the patent to Irie et al. U.S. Pat. No. 5,973,435 permanent magnets are assembled within protective nonmagnetic holders having nonmagnetic metal facings with projections by which they are secured to a non magnetic connecting band. The Kloosterhouse et al. U.S. Pat. No. 5,191,255 shows a permanent magnet mounted in a U-shaped channel having laterally projecting tabs which receive screws for affixing the channel to a rotor.
Accordingly, it is an object of the present invention to provide a magnet retention arrangement for high speed electric machine components which overcomes disadvantages of the prior art.
Another object of the invention is to provide a magnet retention arrangement for high speed machines by which a magnet is securely retained in a rotor operating at high speed without introducing flux path discontinuities.
These and other objects of the invention are attained by providing a bimetallic U-shaped channel which has side walls made of ferromagnetic material to transmit magnetic flux from a magnet within the channel to an adjacent pole piece and a nonmagnetic alloy cap bonded to one edge of each side wall. The side walls of the channel are preferably bonded to the nonmagnetic cap by welding or hot isostatic pressed powder metallurgy techniques and nonmagnetic alloy feet are similarly joined to the opposite edges of the channel walls to secure the channel to the rotor and avoid magnetic shorting and discontinuities. Preferably, the nonmagnetic feet have lateral projections received in corresponding grooves in a nonmagnetic core. One or more permanent magnets are inserted into the magnet retention channel and the assembly is then slidably inserted in the corresponding grooves in the nonmagnetic core. By providing a channel arrangement utilizing such combined magnetic and nonmagnetic channel elements having high strength joints between magnetic and nonmagnetic materials, magnetic flux path discontinuities can be avoided while maximizing magnet retention strength of the channel.